While I've long known that building dofollow backlinks through forum participation benefits site SEO, I just didn't know how much. But this blogger has managed to quantify it somewhat by running an experiment.
His results show it's clearly worth doing, even if the forums you participate in are not related to your site.
Personally, I like participating - particularly on some of the bigger forums - because there's almost always something to provoke a response. So you're not just sitting there looking at a blank screen trying to come up with something new and interesting to say. The process of writing is pleasurable, and you're link building without thinking about it. Also, the forum posts you write can be used again as the basis for articles or blog posts. As long as they are rewritten substantially, they will be unique content.